The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. Furthermore, all embodiments are not necessarily intended to solve all or even any of the problems brought forward in this section.
Inductive loudspeakers often include a coil arranged around a magnetic core which is mechanically coupled with a membrane. Sound is produced by membrane displacements caused by magnetic core motion through inductive coupling to the coil which is controlled by an electrical signal oscillating at given frequencies.
Loudspeakers converting thus an electrical signal into an acoustic signal can be endangered to malfunction or permanent destruction when they are solicited beyond their acceptable limits. If the electrical signal level is too high at specific frequencies, membrane displacement can be such that damage can occur, either by self-heating, mechanical constraint, or by demagnetization of the magnetic core. For instance, the coil of a loudspeaker can hit the mechanical structures of the device or the mobile membrane can be torn if the constraints are too high.
In particular, these issues are very complex to solve for small inductive loudspeakers such as those in mobile devices such as mobiles or smart phones. Dimensions of those loudspeakers impact the heat dissipation and mechanical constraints.
Moreover, being a mechanical oscillator, the loudspeaker may have a resonant frequency which amplifies the amplitude of the control signal at said frequency.
In order to protect inductive loudspeakers against damages due to self-heating and excessive mechanical displacement of the membrane, non adaptive systems have been developed based on an “a priori” prediction of the frequency response of the inductive loudspeakers.
U.S. Pat. Nos. 4,113,983, 4,327,250 and 5,481,617 propose to use variable cut-off frequency filters driven by a membrane displacement predictor. The filter parameters are set according to a prediction of the loudspeaker membrane displacement response over frequency. Parameters are predicted based on a static model of the loudspeaker which is defined once in the life of the product.
U.S. Pat. No. 5,577,126 proposes to use attenuators. The output of the displacement predictor is fed-back into the input signal, according to a feedback parameter computed by a threshold calculator, this parameter being calculated once in the life of the product.
International patent application No. WO 01003466 proposes to use multi-frequency band dynamic range controllers. The input signal is divided into N frequency bands by a bank of band-pass filters. The energy of each frequency band is controlled by a variable gain before being summed together and input to the loudspeaker. A processor monitors the signal level in each frequency band and acts on parameters of each of the variable gain subsystems in order to limit the membrane displacement based on pre-calculated frequency response.
Nevertheless, in case of variations of the loudspeaker transfer function over time, these solutions could not be able to adapt their parameters, as these parameters are calculated once in the life of the product. These variations may result from several factors: temperature, atmospheric pressure, ageing, humidity variations, etc. In contrast, an “a priori” based compensation can not track the real time loudspeaker response, and a compensation filter can not be able to avoid loudspeaker damages in certain conditions.